Four-way change-over valves presently used in air conditioning systems have to accommodate very large pressure differentials, in the order of 30 atmospheres or more. Such high pressure differentials make it difficult to assure that the valve will not leak in its high pressure section, while at the same time to permit change-over from one operating condition to another by the use of a relatively small amount of force. Van Allen U.S. Pat. No. 2,855,000 addresses this problem in a simple manually-operated change-over valve providing only a simple change-over operation. Other four-way change-over valves hereto developed have been of a relatively complicated and expensive construction, as shown for example in U.S. Pat. Nos. 5,462,085 and 5,507,315.
Existing air-conditioning systems are also subject to a number of other problems. One problem is frosting or icing, which can occur when the system is operated in the heating mode (during the winter) or in the cooling mode (during the summer). Should frosting occur in the heating mode, the usual remedy is to change-over to the cooling mode in order to heat the outside coil, and also to shut-off the fan. As a result, considerable energy is lost, and the heating time and the heating capacity are reduced. Should frosting occur in the cooling mode, the usual remedy is to shut-off the compressor and/or to stop or change the speed of the fan, which thereby also involves a loss of energy, time, and cooling capacity. Moreover, interrupting the operation of the compressor is unhealthy to the compressor and requires waiting several minutes before its operation can be resumed. Further, to prevent frosting in the cooling mode, the system is generally designed to operate the evaporator at a temperature significantly above freezing, e.g. about 7.degree. C., to accommodate changes in the outside temperature; this also reduces the efficiency and cooling capacity of the system as compared, for example, when operating at a temperature closer to 0.degree. C.
Another problem involved in present air-conditioning systems is in reducing the cooling or heating capacity of the system, e.g. when the volume of the enclosed space to be cooled or heated is significantly reduced as by shutting off rooms, etc. The present air-conditioning systems are generally merely turned-off in order to reduce the cooling or heating capacity. However, this manner of reducing the capacity also reduces the overall efficiency of the system and wastes energy. Moreover, frequent interruption of the system tends to reduce the useful life of the compressor and the fan.